Introduction and Outline

First, as we approach the end of the course, let’s take a step back. You’ve learned a lot! We’ve focused specifically on the connections between structure and properties, highlighting how different assemblies of atoms at various length scales influence key attributes of materials, such as their mechanical, electrical, optical, and thermal properties.

Now, we’ll broaden our scope and learn how to use the "profile" of a material—its combination of properties—to make decisions about selecting the right material for a specific application. This is a big and important question: the materials we use in various applications significantly influence the performance of the resulting object. For instance, if I’m choosing a bicycle, the material (along with the mechanical design, of course) plays a critical role.

An example: Let’s say I want to buy a bicycle to ride across campus. I’d like it to be easy to ride (i.e., lightweight), but I don’t want to spend a lot of money because I’m worried it might be stolen. What sort of bike should I choose?

As it turns out, the price of a bicycle is largely inversely proportional to its weight, which is determined by the material that its constructed from. Steel bikes are the cheapest but the heaviest. Aluminum bikes are lighter and moderately priced. Titanium bikes are more expensive and can be even lighter, while carbon fiber-reinforced polymer bikes have the highest price point but are the lightest.

So, which should I choose? It depends on how I balance my selection objectives—weight versus cost. Based on my "use case", above, it’s clear that a steel or aluminum bike might be the better choice for me since I’m not racing; I’m just getting around campus.

Materials Selection

In this chapter, we’ll introduce the method of Materials Selection, an essential step in the design process for any physical object. If you’re designing anything tangible, materials selection will likely play a role. The goal is to select a material that maximizes performance. From the first day of class, you’ve understood what this implies: you need to find the material with the optimal (or at least acceptable) combination of properties to achieve the best performance.

Before we dive in, it’s worth noting that this topic is a bit different from others we’ve covered. It focuses on performance optimization through property evaluation and optimization. It’s part materials science, part engineering, and part product design. One of the main contributors to the field of Materials Selection is Michael Ashby, who developed the graphical approach for materials optimization that we’ll cover in this chapter. He also contributed significantly to materials design and materials [ontology](https://en.wikipedia.org/wiki/Ontology_(information_science). Much of the material we’ll explore here is derived from Professor Ashby’s work.

Outline

  • Section 14.3- The Role of Materials Selection in Design: We’ll begin by exploring how materials selection is integral to the design of any physical product. We'll highlight the importance of materials selection in achieving design objectives.
  • Section 14.4 - The Character of a Material—Material Properties Profile: Any material property can influence its performance. Here's we'll briefly recap the array of properties that may need to be considered during a materials selection endeavor.
  • Section 14.5 - Navigating Ashby Diagrams: Ashby diagrams are one of the most useful ways to communicate and explore multiple materials properties at the same time. We'll show you how to navigate and interpret these charts.
  • Section 14.6 - Materials Selection Techniques: Objectives and Criteria: Ultimately, we'll interact with Ashby diagrams in two ways: through maximizing objective functions and by applying selection criteria. We'll present practical examples of these methods in this section.

Outcomes

  • Navigate Ashby diagrams and (broadly) interpret trends/behaviors in materials properties.
  • Utilize and adapt (but don’t derive) performance indices (denoted $M$ or $P$) to aide in the optimal selection of materials for specific applications.
  • Apply selection criteria to define limits for acceptable materials in a specific applications.